Chromatography refers to a set of separation technologies that are used to separate compounds in mixtures. These techniques have a stationary phase and mobile phase. The separation is achieved as the mixture of compounds pass in the mobile phase over the stationary phase and interact with the stationary phase. The difference between the types of chromatography is the nature of stationary phase, mobile phase and the conditions under which the analysis is conducted.

When properly used, chromatography is may help assure that mixtures containing more than one component are consistent. This is important for polymers. The use of a chromatographic fingerprint along with a Fourier transform infrared spectroscopy (FTIR) fingerprint help minimize differences in raw materials that can cause batch problems with formulations.

Both gas liquid chromatography (GLC) and gel permeation chromatography (GPC) are analytical techniques used to separate oligomer mixtures, but they both function by different methodologies.

Gas chromatography (GC) is a chromatographic technique used to analyze a mixture of compounds. A salient characteristic of the technique is that the components can be placed in the vapor phase without degradation. In GC, there are two phases—the mobile phase and the stationary phase. While the mobile phase is a gas such as helium or nitrogen, the stationary phase is a layer of liquid on an inert solid. The mixture of compounds being analyzed are injected in a suitable solvent into a preheated area in which it becomes a gas. The gas enters the column carried by the inert gas. The column is often in a chamber that is heated during the separation. The separation occurs based on the solubility of the compound being analyzed in the liquid coated on the stationary phase.

One example of GLC is the separation of D4 and D5 from ethanol. Another is separation of methyl esters in methyl cocotte.

Gel permeation chromatography (GPC), also called gel exclusion chromatography is a chromatographic technique that separates molecules by their size, rather than the solubility of the compound being analyzed in a solvent on the stationary phase. The GPC analysis is accomplished by passing a solution of the polymer through a porous stationary phase. Larger molecules are excluded from penetrating the stationary phase, while smaller ones penetrate the porous stationary phase, remaining within them for some time.

If the mixture contains molecules that are too small, it may be retained and not seen. There is a limited range of molecular weights that can be separated by each column; therefore, the size of the pores for the packing should be chosen according to the range of molecular weight of compounds to be separated.

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Figure 1. Gas chromatography

The gas enters the column carried by the inert gas. The column is often in a chamber which is heated during the separation. The separation occurs based upon the solubility of the compound being analyzed in the liquid coated on the stationary phase.